1. Field of the Invention
This invention relates to a permanent magnet stepper motor, and in particular to such a motor for controllably moving a rotor to variable angle rotational positions.
2. Description of Related Art
A stepper motor is a versatile device having many different uses. For example, a stepper motor can be operated as a synchronous motor powered by pulses of electrical current in such applications as clocks, meters, timing devices and recorders. Such a motor can also be pulsed at will to effect predetermined increments of movement of elements such as print heads and paper feeders in printers. A stepper motor can also be pulsed at variable speeds to drive apparatus such as a pump which must be operated at variable flow rates.
A typical permanent magnet stepper motor is a one or two phase device comprising a rotor disposed for rotation around a central axis within an annular stator. The rotor includes around its circumference a permanently magnetized region which successively forms a number of North-South pole pairs. The stator includes first and second joined field cups for each phase, each containing a winding of insulated wire surrounding a plurality of magnetizable pole pieces corresponding in number to the magnetic poles in the rotor.
For each phase, the pole pieces of the first field cup are angularly displaced from those of the second field cup to effect a rotational force on the rotor when electrical current is passed through the windings. By controlling the timing and polarities of current pulses passed through the windings, the rotor can be controllably stepped through discrete rotational movements. Further details of typical stepper motor structure, operation and drive circuity can be obtained from a publication entitled AIRPAX Stepper Motor Handbook published by Airpax Corporation, 604 West Johnson Avenue, P.O. Box 590, Cheshire, CT, U.S.A.
One limitation of conventional single phase stepper motors is their inability to vary the rotational positions to which the rotor is stepped. Both the direction and the speed of rotation of the rotor are electrically controllable, but the discrete rotational positions to which the rotor can be positioned are fixed by the positions of the stator pole pieces. In some applications, it is desirable to electrically control the angular separation between the positions to which the rotor can be rotated, as well as the speed and direction of rotation. This capability would enable the user to electrically adjust the angle of rotation to match his needs and to change the angle at will. Some examples of such applications are motors for powering scanner mirrors in laser scanner devices and for powering strip chart recorders.